AN ABNORMAL TSUNAMI GENERATED BY OCTOBER 25 th, 2010 MENTAWAI EARTHQUAKE

Transcription

1 AN ABNORMAL TSUNAMI GENERATED BY OCTOBER 25 th, 2010 MENTAWAI EARTHQUAKE Bambang Sunardi 1, Suci Dewi Anugrah 2, Thomas Hardy 1, Drajat Ngadmanto 1 1 Research and Development Center, Indonesia Meteorological Climatological and Geophysical Agency 2 Tsunami Mitigation, Indonesia Meteorological Climatological and Geophysical Agency ABSTRACT A research of tsunami generated by the October 25 th 2010 earthquake at Mentawai Western of Sumatra has been investigated. The observation of tsunami run up is about m at three locations in the South and North Pagai. Numerical simulation of tsunami using the source mechanism obtained from BMKG results 3.8 m of tsunami wave height, while the propagation model shows that tsunami reach Enggano and Padang for about 38 and 58 minutes close to the tsunami travel time observation. It is clearly showed that the result of run up model is lower than its observation. From the calculation of the magnitude, it is obtained that the tsunami magnitude is about 8.1. This value is higher than the moment magnitude which is only 7.4. It can be conclude that the tsunami Mentawai can be characterized as an abnormal tsunami. This tsunami can also be categorized as a Tsunami Earthquake (TsE). INTRODUCTION Many studies of some great earthquakes in subduction zone of Sumatera had been carried out. Those investigations have contributed significantly on the seismic hazard potential at this area. Natawidjaja, 2007 noted that the potential megathrust earthquake at the area of the subduction depends on the fault segmentation, the dimension of the locked region and the history of the earthquake. These parameters determine the strain energy accumulation area that can generate a big earthquake. Subarya et al, 2006 had been investigated the great earthquake of Aceh- Andaman (2004, Mw 9.15), while Briggs et al, 2006, investigated the Nias Simelue (2005, Mw 8.7). Both of the earthquakes were characterized by the seismic gap zone. The Aceh Earthquake was already signed by the Simeuleu Earthquake of 2002 (Mw7.4), and then the Aceh Earthquake was assumed as an earthquake-triggered of the Nias Earthquake which occurred three months later after that. After a series of two large earthquakes in the northern zone of Sumatra, Natawidjaja et al, 2007 predicted the megathrust of Mentawai Earthquake in the south of Sumatera. This prediction based on a study of paleogeodesy and paleoseismic (Natawidjaja, 2003) that noted the last major earthquake in Mentawai had occurred in the 1300 s and 1600 s, therefore, the cycle of the Mentawai Megathrust Earthquake is about 200 years. Most of the big earthquakes occurred at the Sumatera area generated a tsunami. Jaiswal et al., (2006), noted that there were 33 tsunamis occurred at the Sumatera area. As a part of Sunda Arc, Sumatera had been much more active than Java. Table 1 gives a list of Tsunami Occurrence in Sumatera area. From the list of tsunami events, generally, the tsunamis in the Sumatera area are generated by an earthquake with a magnitude of more than 7 Ms. The Mentawai Earthquake which happened on Oktober 25 th, 2010 is an earthquake that generated a tsunami due to its magnitude and depth. This study investigated a Mentawai Tsunami which generated by an earthquake with a magnitude of 7.2 Mw. The earthquake has

2 occurred along the plate interface boundary between the Australia and Sunda plates at Pagai Selatan Sumatera. According to BMKG, the earthquake located at the location of 3.6 o S and 99.9 o E with 10 km depth. This big earthquake occurred due to the movement of the Australia Plate with respect to the Sunda Plate with a velocity of approximately mm/yr. The Mamoru Nakamura s Program was applied in this study to run a modeling of the Mentawai tsunami. A field study to the Mentawai Islands after the event had also been carried out in this research to provide the height of tsunami run-up in that area to validate the tsunami modeling. TECTONIC SETTING The west Sumatera region is a part of the Eurasian Plate with a very slow speed of approximately 0.4 cm/year that moves relatively to the southeast. An interaction between The Eurasian and the Indian Ocean plate is occurred in the western part of this province that moved to the north at speeds up to 7 cm / year relatively (Minster and Jordan, 1978 in Yeats et al., 1997). This interaction produces an oblique subduction, which had been formed since the Cretaceous era and still continues up to now. In addition to subduction, two plates of this interaction also resulted in major structural pattern of Sumatra, which are known as the Sumatra Fault Zone and Mentawai Fault Zone (Figure 1). The Tectonic evolution of West Sumatra before the Age of Neogen tectonics was characterized by the expansion (Tectonic rifting) followed by the collision, amalgamation, and akrasi which lead to the formation of mountains, crumpling, and faulting (Simanjuntak, 2004). The Unveiling of melange rock in North Sumatra and West Sumatra of the Cretaceous age indicates the presence of subduction related to the complex akrasi systems (Asikin, 1974; Simanjuntak, 1980; Sukamto, 1986; Wajzer et al, 1991 in Simandjuntak, 2004). In the Age of Paleogene subduction system was relatively shifted to the west. It was proved by the presence of mélange rocks at Nias Island, Pagai and Sipora which are located in west of Sumatra Island (Katili, 1973; Karig et al., 1978; Hamilton, 1979; Djamal et al., 1990 ; Andi- Mango, 1991 in Simandjuntak, 2004). The change of Melange rock lane associated with akrasi complex is known as the rollback. The Orogenesaon process in Neogen Age produces the existence of the four phenomenons in this region namely the Bukit Barisan Mountains, an oblique subduction with angle range from 50 o 65 o in the west of Sumatera, the Sumatera fault, and the Sumatra magmatisme activities (Barber, 2005). The western part of Sumatera Island is an area located on the outskirts of the active plate that is reflected by a high frequency occurrence of the earthquakes. The earthquakes distribution in this region is not only caused by the activity of subduction zones, but it also caused by an active fault systems along the island of Sumatra. Based on the Harvard CMT focal mechanism data, most of the subduction zone seismicity shows a normal fault type, while most of seismicity activity on the ground shows a mechanism of a strike slip fault (Figure 2). DATA AND TSUNAMI MODELING To calculate run-up heights we use a code that is modeled by Nakamura. This code applied a finitedifference method. The bathymetry data are obtained from ETOPO2 provided by National Geophysical Data Center. The study area is showed in figure 3. The grid interval of bathymetry was 2.5 km x 2.5 km. In this simulation we used 5 seconds time step used for calculations. A focal mechanism solution from BMKG was used for this model. An uplift red fault block (Figure 4) represents an earthquake source mechanism that reflects an oblique reverse fault. This source mechanism triggered a tsunami after the earthquake occurrence. According to the earthquake source of BMKG data it was obtained that the maximum vertical displacement is 1.3 m. Table 2 is the simulation parameters as a solution of focal mechanism. The numerical simulation of tsunami propagation shows that tsunamis arrived the coastlines area of Purourogat and Munte, Enggano, and Padang about 11, 38 and 58 minute after the earthquake respectively (Figure 5).

3 The run-up simulations are showed on the Figure 6. The maximum tsunami height at the Pagai Island is about 4 m. Figure 6b shows a tsunami height at three locations, while Table 3 is a list of tsunami height at many points along the coast. It appears that a bay or an estuary experiences a higher run-up value than its surrounding area relatively. FIELD SURVEY Post-tsunami field survey was also conducted in this study. We measured and collected as many as possible data of tsunami height traces, tsunami direction, inundation, and also the impact of tsunami on life and material losses. Three locations were observed in that survey namely Munte Kecil, Purourogat, and Muntei. The first and two are located at Malakopak, South of Pagai, and the third are located at Betumonga, North of Pagai. The followings are a brief description of observation result at each area. The first observation point was in the Dusun Munte Kecil, Desa Malakopak, South Pagai. We found 4 buildings were damage at the distance of 70 m, 120 m, 150 m, and 170 m from the beach respectively. At a distance of 230 m from the beach, some buildings were found safe. We did an interview with some local residents to investigate whether any casualties due to the tsunami. They said that there were no casualties caused by the tsunami. After the earthquake of 2007, the government had relocated the coast resident to the safe place. This effort, however, had saved the people from tsunami hazard successfully. Because of the earthquake, this area can be categorized as a zone of 2-3 MMI scales (weak shocks). We measured approximately 6.4 m of run-up traces, 290 m of inundation, and N15E of tsunami direction. The second one was in the di Dusun Purourogat, Desa Malakopak, South Pagai. This place is a bay area. We found 3 damaged buildings at the location of 50, 120, and 130 m from the beach. At a distance of about 210 m, we found some houses with a small scale of damage. Not far from this location, there was a valley with a depth of about 2.3 m. The tsunami measurement results are about 7.4 m of tsunami run-up, 420 m of tsunami inundation, and N85E of tsunami direction. 71 people were reported die and 4 people were missing. The last observation point was in Dusun Muntei, Desa Betumonga, North Pagai. Most area of Muntei is located at a bay, while some part is located at the edge of an estuary. Although the Muntei area is more far from the earthquake source compare to others, this area was the most tsunami affected. A lot of houses were damaged because the buildings were built close to a sloping beach and confronted the sources of earthquakes directly. The geographic condition of Muntei as a bay and an estuary is another factor that caused this area is more vulnerable than others. The tsunami run-up is higher and its inundation is widespread. The tsunami measurement results are about 5.7 m of tsunami run-up, 406 m of tsunami inundation, and N10E of tsunami direction. 115 people were reported die and 34 people were missing. 73 houses were severely damaged. This study estimated also a tsunami source magnitude based on the wave height distribution at various places in relation with the source of tectonic earthquake which is known as Tsunami Magnitude (Mt). Abe (1979, 1981, 1989b), formulated a calculation of Mt as follows: Mt = logδ + logh where H is the maximum amplitude of tide gauge observation, and Δ is the distance of the earthquake source to the tide gauge. In this case 8.1 of Mt were estimated. MODEL VALIDATION The ocean modeling of tsunami propagation was nearly appropriate when verified with tide gauge observation data. Table 5 shows the comparison of tsunami travel time between observational data (tide gauge) and numerical simulation results of tsunami propagation using Mamoru Nakamura's Program. The model estimated that the tsunamis entered the Pagai island coastline of about 11 minutes after the main earthquake. Therefore, it is not true that the tsunami occurred after BMKG BMKG end tsunami warning at 51 minutes after the earthquake.

4 DISCUSSION AND CONCLUSION The results of post-tsunami survey at the three observation points namely Munte kecil, Purourogat and Muntei noted that the run-up varied between m. The maximum run-up was occurred in Purourogat of about 7.4 m which captured 420 m of inundation area from the beach. In this location the tsunami moved from the direction of N850E. The Munte Kecil and Muntei experienced the maximum run-up of about 6.4 m and 5.7 m respectively, with inundation area of about 290 m and 420 m. In that points the tsunami moved from the direction of N150E and N100E. The geographic condition of the beach influenced the height of tsunami run-up. A bay as well as an estuary, like the shape of the Purourogat beach, will produce a higher tsunami run-up. The run-up estimation of the tsunami numerical model using the BMKG data was 4 m, lower than field measurements of 7.4 m. However, the ocean modeling of tsunami propagation was almost similar with the tide gauge data. It was estimated that within 11 minutes after the main earthquake, the tsunamis began to enter some coastlines area in the South and North of Pagai. The Mentawai tsunami has a similar characteristic with the event of Pangandaran Both of those can be categorized as the TsE, considering that the estimated value of the tsunami run-up was much smaller than the actual height of the tsunami. A slow ground shaking had made tsunami magnitude calculation (Mt) of Mentawai Earthquake was much larger than the earthquake magnitude (Mw). The Mentawai tsunami might be also influenced by other mechanisms. The source of the earthquake was at the point that close to the ocean trench where subduction between the Indo- Australian plate with the Eurasian plate in the area where the accumulation of sediments experiences a great pressure forming a continuous ridge submarine elongated with steep slopes that tend to shock and vulnerable to earthquake shocks. Possible lifting of sediment above it or a large landslide that occurred after the earthquake as an additional factor that triggered the tsunami height becomes abnormal (much higher than normal calculation). However, to prove the source of research takes a more in-depth process. REFERENCES Abe, K., Size of great earthquakes of inferred from tsunami data. J. Geophys. Res., v. 84, pp Abe, K., Size of tsunamigenic earthquakes of the northwestern Pacific, Phys. Earth Planet. Inter., v. 27, pp Abe, K., 1989b. Quantification of tsunamigenic earthquakes by Mt scale, Tectonophys. 166, Barber, A. J., Crow, M. J., Milsom, J. S., Sumatera Geology, Resources and Tectonic Evolution. Geological Society Memoir No. 31, The Geological Society, London, 290 p. Briggs, R., Sieh, K., Meltzer, A.S., Natawidjaja, D., Galetzka, J., Suwargadi, B., Hsu, Y.J., Simons, M., Hananto, N., Suprihanto, I., Prayudi, D., Avouac, J.-P., Prawirodirdjo, L., and Bock, Y. (2006). Deformation and slip along the Sunda megathrust in the Great 2005 Nias-Simeulue earthquake.: Science, v. 311, p Jaiswal, R.K., B.K (2006). Tsunamigenic sources in the Indian Ocean. Lasitha, S., Radhakrishna, M., Sanu, T. D., Seismically active deformation in the Sumatera Java trench arc region : geodynamic implications. Current Science, 90 (5), pp Natawidjaja, D.H. (2003). Neotectonics of the Sumatran fault and paleogeodesy of the Sumatran subduction zone. Ph.D thesis, California Institute of Technology (Caltech). Natawidjaja. (2007). Tectonic Setting Indonesia dan Pemodelan Sumber Gempa dan Tsunami. Presented on Pelatihan pemodelan run-up tsunami, ristek, Agustus 2007, Jakarta. Natawidjaja, D., Sieh, K., Galetzka, J., Suwargadi, B., Cheng, H., and Edwards, R. (2007). Interseismic deformation above the Sunda megathrust recorded in coral microatolls of the

5 Mentawai Islands, West Sumatra: J. Geophys. Res. Simandjuntak, T. O., Tektonika. Publikasi Khusus, Pusat Penelitian dan Pengembangan Geologi, 216 p. Subarya, C., Chlieh, M., Prawirodirdjo, L., Avouac, J.P., Bock, Y., Sieh, K., Meltzner, A.J., Natawidjaja, D.H., and McCaffrey, R. (2006). Plate-boundary deformation associated with the great Sumatra-Andaman earthquake: Nature, v. 440, p Yeats, R. S., Sieh, K., and Allen, C. R., The Geology of Earthquakes. Oxford university press, 567 p.

6 No Year Location Latitude Longitude Magnitude Sumatera Sw Sumatera Sw. Sumatera Se. Sumatera Bengkulu, Sumatera Bengkulu, Sumatera Sw. Sumatera Banda Aceh Sw. Sumatera Sw. Sumatera Sibolga, Sumatera Sw. Sumatera Sw. Sumatera Sw. Sumatera Sw. Sumatera Sw. Sumatera Sumatera Sw. Sumatera Sumatera Sw. Sumatera Sw. Sumatera Sumatera W. Coast Of S. Sumatera Lhoknga, Aceh Sw. Sumatera Sw. Sumatera Sw. Sumatera Sw. Sumatera Off West Coast Of Sumatera Southern Sumatera Off West Coast of Sumatera Off West Coast of Sumatera Off West Coast Of Sumatera Kepulauan Mentawai Table 1: Tsunami catalogue for Sumatera area (Rastogi and Jaiswal, 2006).

7 Simulation Parameters Center Fault Coordinate Length (km) 74.5 Width (km) 26.5 Slip (m) 3.3 Mw 7.4 Latitude -3.2 Longitude 100 Table 2: Simulation Parameters. Longitude Latitude Run Up (m)

8 Longitude Latitude Run Up (m)

9 Longitude Latitude Run Up (m) Table 3: Tsunami height modeling at many points along the Pagai coast. Location Munte Kecil South Pagai Purourogat South Pagai Munte North Pagai Position S E S E S E Direction N15 0 E N85 0 E N10 0 E Run-Up (m) Inundation (m) Table 4: Post Tsunami Survey.

10 Locations Tide Gauge Travel Time (Minute) Simulation Travel Time (Minute) Enggano Padang Table 5: The comparison of tsunami travel time between observational data (tide gauge) and numerical simulation results of tsunami propagation using Mamoru Nakamura's Program. Location Munte Kecil Malakopak South Pagai Purourogat Malakopak South Pagai Munte Betumonga North Pagai Position Run Up Observational (m) Simulation Position Run Up Simulation (m) S E S E S E S E S E S E 2.5 Table 6: Comparison of tsunami run up between observational and numerical simulation. Name Lat Long Distance Arival Time (UTC) Travel Time (minute) Max Water Height (m) Enggano : Padang : Tanabalah : Telukdalam : Table 7: Observational data (tide gauges data).

11 Figure 1: Tectonic of Sumatra and Java (Lasitha et al., 2006). Figure 2: Earthquakes source mechanism in Sumatra and surrounding based on Harvard CMT data (Lasitha et al., 2006).

12 Figure 3: Research area. Figure 4: Source modeling.

13 Figure 5a: Ocean modeling at 11 minutes. Figure 5b: Ocean modeling at 38 minutes. Figure 5c: Ocean modeling at 58 minutes.

14 Figure 6a: Run up modeling. Figure 6b: Run up modeling at Muntei Kecil, Purourogat and Munte.